Antimatter mysteries 3: Does antimatter fall up?

Gravity, we think, works the same way on all matter. But what about antimatter?

AEGIS, a CERN experiment that has just been given the go-ahead, is designed to find out. Gravity is a relatively weak force, so the experiment will use uncharged particles to prevent electromagnetic forces drowning out gravitational effects. It will first build highly unstable pairings of electrons and positrons, known as positronium, then excite them with lasers to prevent them annihilating too quickly. Clouds of antiprotons will rip these pairs apart, stealing their positrons to create neutral antihydrogen atoms.

Pulses of these anti-atoms shot horizontally through two grids of slits will create a fine pattern of impact and shadow on a detector screen. By measuring how the position of this pattern is displaced, the strength – and direction – of the gravitational force on antimatter can be measured.

Advertisement

It’s a clever idea, but the devil is in the detail, says AEGIS spokesman Michael Doser. “No one has ever made controlled positronium like this, nobody has ever made a positronium excited state with lasers in an environment like this and nobody has ever made an antihydrogen pulse like this.”

If the researchers succeed, it will be well worth the effort. If gravity does affect antimatter differently, it will tell us something not just about antimatter but also about the fundamental theories that underpin modern physics. Einstein’s general relativity, the currently favoured theory of gravity, tells us that the force should work identically on any type of matter. Equally, the standard model predicts that matter and antimatter are identical to all intents and purposes. “If we find that either of these things differ,” says Landua, “then we have found something extremely important.”

Doser is hedging his bets. “I’ll wager a crate of champagne that we won’t see a difference,” he says. “But I’d gladly lose that crate.”